Substrate media such as paper is prone to having the leading and or trailing edges curl up such that the sheet does not lie flat across its entire surface. Media curl is frequently considered one of the root causes of media jams in media handling and registration. Media curl can be induced by several factors such as, for example, relative humidity, media weight, media size, sides imaged or the amount of data contained with a particular digital image.
An important dimension associated with an inkjet printing process, including solid ink jet (SIJ) printing process, is the small print gap between the ink jet heads and the receiving media or drum in the case of drum printing systems. This print gap is on the order of 0.5 mm, and must be tightly controlled to maintain accurate drop placement which in turn results in acceptable image quality. In a web-based media handling system, one can maintain this print gap relatively easily with the proper geometry and web tension. However, for cut-sheet SIJ systems, the sheet edges pose a more difficult problem to solve, as the edges can be lifted up from a media transport due to curl thereby increasing the height of the media. Up-curl designates a direction of the curl towards the print head and down-curl away from the print head. The up and down directions are thus not with respect to gravity, but rather with respect to the print head. Also note that no assumption is made relative to the angle of the mounting of the print head
If these curled edges result in a media height greater than the print gap, the media will come into contact with the print heads during operation, and damage to the jets could occur. As the print heads are expensive, this scenario negatively impacts run costs.
Therefore, it is desirable not to have any or a very minimal amount of up-curl (curl toward the print head). For this purpose a sheet curler is typically used upstream of the print head area. Media curlers are used to remove or impose a certain amount of curl. Media is transported onto the hold-down transport using a traditional nip based registration transport with nip releases. For satisfactory operation, the amount of media curl to be removed is input to the curler control which adapts curler setting based on input curl and desired output curl. Upstream of the registration transport a sheet curler imparts a known amount of curl to the sheet to make sure no or minimal up-curl is present after the sheet is acquired by a hold-down transport. In order to achieve this, a measurement of the maximum sheet height is desirable. Then, the curl controller can modify curler settings to obtain no or minimal up-curl. Common input curl measurements report the amount of curl in only one or two locations across the leading edge (“LE”) or trailing edge (“TE”) of a sheet. However, these one or two measurements typically do not represent the maximum curl of the sheet and hence do not give an accurate amount of curl to be removed be the curler.
Accordingly, it would be desirable to provide a device and method for accurately and efficiently measuring media curl.